Hydroponic cultivation has several advantages over traditional soil cultivation. In particular, the risk from persistent soil-borne diseases is minimised, there is no damage as a result of repetitive cultivation in the same area and the cultivation crop cycle is shorter. Accordingly, hydroponic cultivation has been expanding rapidly over recent years. Nearly all salad crops commercially grown in the UK are produced in glasshouses by a hydroponic system.
There are two systems for the hydroponic cultivation of plants. Typically plants are grown on a large amount of substrate, such as slabs or blocks of mineral fibre. The slabs or blocks are drip fed intermittently with a nutrient solution. The particular feeding programme depends on conditions affecting rates of nutrient utilisation and water uptake.
Alternatively, the plants are grown by a method known as nutrient film technique (NFT) wherein plants are taken from the propagation block in which they are supplied, and suspended so that their roots are in contact with fast flowing nutrient solution.
For economic use of nutrients, and because of regulatory factors affecting the disposal of nutrient-rich water, the water carrying the nutrients needs to be recycled. This may be carried out continuously or, for instance, by collecting the liquid medium in batches, for instance, one thousand liter batches, and then processing the batches for re-use. Since about 70 percent of the inflowing liquid is retained by the plants, or lost by evaporation and respiration, reprocessing the outflowing liquid effectively involves dilution and re-adjustment of nutrient levels.
Although the hydroponic nutrient solutions contain no organic source of carbon when applied to the plants, the outflowing liquid accumulates organic exudates from the plant roots and also possibly from algae that grow on the well lit surfaces of the growth blocks. As a result, carbon-containing nutrients build up on which plant-pathogenic fungi can grow. Accordingly, a major problem for those operating a recycling hydroponic system is the control of fungal pathogens, and particularly their spores, which tend to accumulate in the liquid and hence constantly re-infect the plants.
The above mentioned problem is currently being tackled by attempting to sterilise the nutrient liquid before it is recycled. Sterilisation methods include pasteurisation, hollow-fibre ultrafiltration, UV-irradiation and chemical treatments using, for instance, ozone or peracetic acid. However, such methods are expensive and they also eliminate a potentially beneficial group of bacteria which may help to control fungal pathogens.